Grind the blackberries, green grapes, and cucumber slices at the bottom of a cocktail shaker. Add some ice, the elderflower syrup, lime juice, basil, and the sweet vermouth, then shake vigorously. Serve in a wine glass over ice, then add soda and Prosecco to taste. Garnish with a basil leaf.
Created by Florence Doyon-Simon, finalist, Made With Love 2018 (Montreal).